Quantum dot light-emitting diode and method of fabricating the same

ABSTRACT

Disclosed is a quantum dot light-emitting diode including a positive electrode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and a negative electrode, wherein the hole injection layer is a p-type oxide semiconductor represented by Formula 1 below:
 
Cu 2 Sn 2-X S 3 —(Ga X ) 2 O 3 ,  [Formula 1]
         wherein X is greater than 0.2 and less than 1.5 (0.2&lt;x&lt;1.5).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. application Ser.No. 16/571,568, filed on Sep. 16, 2019, which claims priority to KoreanPatent Application No. 10-2018-0110935, filed on Sep. 17, 2018 in theKorean Intellectual Property Office, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates to a quantum dot light-emitting diode anda method of fabricating the same, and more particularly, to a quantumdot light-emitting diode using a p-type oxide semiconductor includingCu₂SnS₃—Ga₂O₃ and a method of fabricating the quantum dot light-emittingdiode.

Description of the Related Art

Recently, development of high-performance quantum dot light-emittingdiodes has been actively carried out. In implementing a high-performancequantum dot light-emitting diode, hole transport is considered to be avery important process.

In conventional quantum dot light-emitting diodes, apoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)layer having high conductivity is generally used as a hole injectionlayer.

However, when PEDOT:PSS is used as a hole injection layer, annealingtime is required, resulting in a longer process time. In addition, sincePEDOT:PSS is strongly acidic, the surface of an ITO electrode may bedamaged, and the stability of a device may be deteriorated.

Therefore, studies are underway to use an oxide semiconductor as thehole injection layer.

Oxide semiconductors are suitable for realizing a transparent displaydue to high mobility and transparency thereof. In addition, since oxidesemiconductors have an amorphous or polycrystalline structure at roomtemperature, a separate heat treatment process for forming grains isunnecessary. Thus, oxide semiconductors exhibit excellent propertieswhen applied to a quantum dot light-emitting diode.

In addition, oxide semiconductors are direct semiconductors having highmobility (1 to 100 cm²/Vs) and high band gaps. Unlike silicon-baseddevices, since there is no oxidation phenomenon in oxide semiconductors,oxide semiconductors have an advantage of less variation in devicecharacteristics.

However, oxide semiconductors generally exhibit n-type characteristicsdue to oxygen vacancies and zinc interstitials. Thus, it is difficult toperform p-type doping in oxide semiconductors.

As described above, most oxide semiconductors known to date exhibitn-type characteristics. When a transparent oxide semiconductor havingp-type characteristics is implemented, the transparent oxidesemiconductor may be advantageously used as the hole injection layer ofa quantum dot light-emitting diode. Therefore, it is necessary todevelop a p-type transparent oxide semiconductor through optimization ofdoping conditions or development of new materials.

RELATED DOCUMENTS Patent Documents

-   Korean Patent Application Publication No. 10-2016-0030767, “ORGANIC    LIGHT-EMITTING DIODE USING P-TYPE OXIDE SEMICONDUCTOR INCLUDING    GALLIUM AND METHOD OF PREPARING THE ORGANIC LIGHT-EMITTING DIODE”-   Korean Patent Application Publication No. 10-2012-0009229, “THIN    FILM TRANSISTOR AND METHOD OF PREPARING THE SAME”

Non-Patent Documents

-   Dominik M. Berg et al., “Thin film solar cells based on the ternary    compound Cu₂SnS₃” (May 31, 2012)

SUMMARY OF THE DISCLOSURE

Therefore, the present disclosure has been made in view of the aboveproblems, and it is an object of the present disclosure to provide ahigh-efficiency quantum dot light-emitting diode in which a p-type oxidesemiconductor including Cu₂SnS₃—Ga₂O₃ is used as a hole injection layer.

It is another object of the present disclosure to provide a quantum dotlight-emitting diode in which a p-type oxide semiconductor fabricatedusing a solution process is used. According to the present disclosure,since a p-type oxide semiconductor fabricated using a solution processis used, the quantum dot light-emitting diode is applicable to alow-temperature process, thereby reducing preparation costs.

In accordance with one aspect of the present disclosure, provided isquantum dot light-emitting diode including a positive electrode, a holeinjection layer, a hole transport layer, a light-emitting layer, anelectron transport layer, and a negative electrode, wherein the holeinjection layer is a p-type oxide semiconductor represented by Formula 1below:Cu₂Sn_(2-X)S₃—(Ga_(X))₂O₃,  [Formula 1]

wherein X is greater than 0.2 and less than 1.5 (0.2<x<1.5).

The p-type oxide semiconductor may be heat-treated or treated withultraviolet light/ozone.

The heat treatment may be performed at 150 to 250° C.

The heat treatment may be performed for 10 to 90 minutes.

The ultraviolet light/ozone treatment may be performed for 0 to 5minutes.

In accordance with another aspect of the present disclosure, provided isa method of fabricating a quantum dot light-emitting diode including astep of forming a positive electrode on a substrate; a step of forming ahole injection layer on the positive electrode; a step of forming a holetransport layer on the hole injection layer; a step of forming alight-emitting layer on the hole transport layer; a step of forming anelectron transport layer on the light-emitting layer; and a step offorming a negative electrode on the electron transport layer, whereinthe hole injection layer is formed by forming a film using a solutionprepared by mixing a p-type oxide semiconductor represented by Formula 1below and a solvent:Cu₂Sn_(2-X)S₃—(Ga_(X))₂O₃,  [Formula 1]

wherein X is greater than 0.2 and less than 1.5 (0.2<x<1.5).

The p-type oxide semiconductor may include a step of preparing aprecursor solution containing Cu, S, M, and Ga, wherein M includes oneor more compounds selected from SnO, ITO, IZTO, IGZO, and IZO; a step offorming a coating layer by applying the precursor solution onto thesubstrate; and a step of heat-treating the coating layer.

The solvent may be prepared by mixing 2-methoxyethanol, ethylene glycol,and 5 to 50 volume percent of at least one of acetonitrile, DI water, analcohol, cyclohexane, toluene, and a quantum dot solvent.

The p-type oxide semiconductor may be heat-treated or treated withultraviolet light/ozone.

The heat treatment may be performed at 150 to 250° C.

The heat treatment may be performed for 10 to 90 minutes.

The ultraviolet light/ozone treatment may be performed for 0 to 5minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent disclosure will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view of a quantum dot light-emitting diodeaccording to one embodiment of the present disclosure;

FIG. 2A is a graph showing Raman spectra for an untreated Cu₂SnS₃—Ga₂O₃thin film (black line) and an ultraviolet light/ozone-treatedCu₂SnS₃—Ga₂O₃ thin film (gray line);

FIG. 2B is a graph showing the X-ray diffraction (XRD) patterns ofCu₂SnS₃—Ga₂O₃ thin films heat-treated at 160° C. (black) and 200° C.(gray), respectively;

FIG. 2C is a graph showing the light transmittance of a Cu₂SnS₃—Ga₂O₃thin film depending on thickness, and FIG. 2D shows Tauc plots formeasuring band gaps;

FIG. 2E is a graph showing the light transmittance of a Cu₂SnS₃—Ga₂O₃thin film depending on the presence or absence of ultravioletlight/ozone treatment, and FIG. 2F shows Tauc plots for measuring bandgaps;

FIG. 2G is a graph showing the light transmittances of Cu₂SnS₃—Ga₂O₃thin films having molar ratios of Sn to Ga (Sn:Ga) of 1.8:0.2, 1.5:0.5,1.0:1.0, and 0.5:1.5, respectively, and FIG. 2H shows Tauc plots formeasuring band gaps;

FIG. 2I is an atomic force microscope (AFM) image of a Cu₂SnS₃—Ga₂O₃thin film, FIG. 2J is a scanning electron microscope (SEM) image of aCu₂SnS₃—Ga₂O₃ thin film, FIG. 2K is a high-resolution transmissionelectron microscope (HRTEM) image of a Cu₂SnS₃—Ga₂O₃ thin film, and FIG.2L shows a crystal structure of a selected region obtained by electrondiffraction;

FIG. 3A shows energy-dispersive spectrometer (EDX) spectra for analyzingthe chemical composition of a Cu₂SnS₃—Ga₂O₃ thin film according to anembodiment of the present disclosure;

FIGS. 3B to 3F are graphs showing the results of X-ray photoelectronspectroscopy (XPS) analysis for analyzing the chemical composition ofthe surface of a Cu₂SnS₃—Ga₂O₃ thin film treated with ultravioletlight/ozone for 2 minutes and heat-treated at 200° C.;

FIGS. 4A to 4C are contour plots for carrier concentration, Hallmobility, and resistivity depending on heat treatment temperature andheat treatment time in a Cu₂SnS₃—Ga₂O₃ thin film treated withultraviolet light/ozone for 2 minutes according to an embodiment of thepresent disclosure, and FIGS. 4D to 4F are error bar plots for carrierconcentration, Hall mobility, and resistivity;

FIGS. 5A to 5C are contour plots for carrier concentration, Hallmobility, and resistivity depending on ultraviolet light/ozone treatmenttime and molar ratios of Sn to Ga (Sn:Ga) in a Cu₂SnS₃—Ga₂O₃ thin filmheat-treated at 200° C. for 60 minutes according to an embodiment of thepresent disclosure, and FIGS. 5D to 5F are error bar plots for carrierconcentration, Hall mobility, and resistivity;

FIGS. 6A and 6B show a schematic configuration of a Cu₂SnS₃—Ga₂O₃ thinfilm according to an embodiment of the present disclosure consisting ofcrystalline Cu₂SnS₃ and amorphous Ga₂O₃, and show current flow throughthe Cu₂SnS₃—Ga₂O₃ thin film;

FIGS. 7A to 7D show the He (I) ultraviolet photoelectron spectroscopy(UPS) spectra of PEDOT:PSS or a Cu₂SnS₃—Ga₂O₃ thin film deposited on anITO substrate according to an embodiment of the present disclosure;

FIG. 8A shows UPS spectra in the secondary electron cut-off regions of aCu₂SnS₃—Ga₂O₃ thin film measured at various negative biases, and FIG. 8Bshows UPS spectra in the VB edges of the Cu₂SnS₃—Ga₂O₃ thin film;

FIG. 9A is a cross-sectional view of a quantum dot light-emitting diodeaccording to an embodiment of the present disclosure, and FIG. 9B is across-sectional view of a TEM image of the quantum dot light-emittingdiode;

FIG. 9C is an HRTEM image of green quantum dots according to anembodiment of the present disclosure, and FIG. 9D illustrates energyband diagrams of quantum dot light-emitting diodes including varioushole injection layers;

FIG. 10A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode including aCu₂SnS₃—Ga₂O₃ thin film according to an embodiment of the presentdisclosure depending on ultraviolet light/ozone treatment time, FIG. 10Bis a graph showing the luminance-voltage (L-V) characteristics of thequantum dot light-emitting diode, and FIG. 10C is a graph showing thecurrent efficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode;

FIG. 11A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode including aCu₂SnS₃—Ga₂O₃ thin film according to an embodiment of the presentdisclosure depending on heat treatment temperatures, FIG. 11B is a graphshowing the luminance-voltage (L-V) characteristics of the quantum dotlight-emitting diode, and FIG. 11C is a graph showing the currentefficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode;

FIG. 12A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode including aCu₂SnS₃—Ga₂O₃ thin film according to an embodiment of the presentdisclosure depending on molar ratios of Sn:Ga, FIG. 12B is a graphshowing the luminance-voltage (L-V) characteristics of the quantum dotlight-emitting diode, and FIG. 12C is a graph showing the currentefficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode;

FIG. 13A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode according to anembodiment of the present disclosure depending on the thicknesses of aCu₂SnS₃—Ga₂O₃ thin film, FIG. 13B is a graph showing theluminance-voltage (L-V) characteristics of the quantum dotlight-emitting diode, and FIG. 13C is a graph showing the currentefficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode;

FIGS. 14A to 14E are graphs showing the characteristics of a quantum dotlight-emitting diode including a PEDOT:PSS-based hole injection layerand a quantum dot light-emitting diode including a Cu₂SnS₃—Ga₂O₃ thinfilm-based hole injection layer having a molar ratio of Sn to Ga (Sn:Ga)of 1:1 according to an embodiment of the present disclosure; and

FIG. 15A is a graph showing EL intensity-current density depending onwavelength, and FIG. 15B shows a CIE color specification system.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure will now be described more fully with referenceto the accompanying drawings and contents disclosed in the drawings.However, the present disclosure should not be construed as limited tothe exemplary embodiments described herein.

The terms used in the present specification are used to explain aspecific exemplary embodiment and not to limit the present inventiveconcept. Thus, the expression of singularity in the presentspecification includes the expression of plurality unless clearlyspecified otherwise in context. It will be further understood that theterms “comprise” and/or “comprising”, when used in this specification,specify the presence of stated components, steps, operations, and/orelements, but do not preclude the presence or addition of one or moreother components, steps, operations, and/or elements thereof.

It should not be understood that arbitrary aspects or designs disclosedin “embodiments”, “examples”, “aspects”, etc. used in the specificationare more satisfactory or advantageous than other aspects or designs.

In addition, the expression “or” means “inclusive or” rather than“exclusive or”. That is, unless otherwise mentioned or clearly inferredfrom context, the expression “x uses a or b” means any one of naturalinclusive permutations.

In addition, as used in the description of the disclosure and theappended claims, the singular form “a” or “an” is intended to includethe plural forms as well, unless context clearly indicates otherwise.

Although terms used in the specification are selected from termsgenerally used in related technical fields, other terms may be usedaccording to technical development and/or due to change, practices,priorities of technicians, etc. Therefore, it should not be understoodthat terms used below limit the technical spirit of the presentdisclosure, and it should be understood that the terms are exemplifiedto describe embodiments of the present disclosure.

Also, some of the terms used herein may be arbitrarily chosen by thepresent applicant. In this case, these terms are defined in detailbelow. Accordingly, the specific terms used herein should be understoodbased on the unique meanings thereof and the whole context of thepresent disclosure.

Meanwhile, terms such as “first” and “second” are used herein merely todescribe a variety of constituent elements, but the constituent elementsare not limited by the terms. The terms are used only for the purpose ofdistinguishing one constituent element from another constituent element.

In addition, when an element such as a layer, a film, a region, and aconstituent is referred to as being “on” another element, the elementcan be directly on another element or an intervening element can bepresent.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art. It will be further understood that terms,such as those defined in commonly used dictionaries, should beinterpreted as having a meaning that is consistent with their meaning inthe context of the relevant art and the present disclosure, and will notbe interpreted in an idealized or overly formal sense unless expresslyso defined herein.

In addition, in the following description of the present disclosure, adetailed description of known functions and configurations incorporatedherein will be omitted when it may make the subject matter of thepresent disclosure unclear. The terms used in the specification aredefined in consideration of functions used in the present disclosure,and can be changed according to the intent or conventionally usedmethods of clients, operators, and users. Accordingly, definitions ofthe terms should be understood on the basis of the entire description ofthe present specification.

Hereinafter, a quantum dot light-emitting diode according to oneembodiment of the present disclosure will be described with reference toFIG. 1 .

FIG. 1 is a cross-sectional view of a quantum dot light-emitting diodeaccording to one embodiment of the present disclosure.

The quantum dot light-emitting diode according to one embodiment of thepresent disclosure may include a positive electrode 110, a holeinjection layer 120, a hole transport layer 130, a light-emitting layer140, an electron transport layer 150, and a negative electrode 160.

Referring to FIG. 1 , in the quantum dot light-emitting diode accordingto one embodiment of the present disclosure, the positive electrode 110is formed on a substrate (not shown).

The substrate is a base substrate for forming a quantum dotlight-emitting diode. Substrates generally used in the art to which thepresent disclosure pertains may be used as the substrate of the presentdisclosure. In addition, the material of the substrate is notparticularly limited, and may include silicon, glass, plastic, metalfoil, and the like.

For example, the plastic substrate may include polyethyleneterephthalate (PET), polyethylenenaphthelate (PEN), polypropylene (PP),polycarbonate (PC), polyimide (PI), tri acetyl cellulose (TAC), andpolyethersulfone (PES), and a flexible substrate such as an aluminumfoil or a stainless steel foil may be used.

The positive electrode 110 is an electrode for providing holes to adevice, and may be formed by performing a solution process such asscreen printing on a transmissive electrode, a reflective electrode, ametal paste, or a metal ink material in a colloid state in apredetermined liquid.

The transmissive electrode material may include at least one of indiumtin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO₂), zinc oxide(ZnO), multilayer metal oxide/metal/metal oxide, graphene, and carbonnanotube, which are transparent and have excellent conductivity.

The reflective electrode material may include at least one of magnesium(Mg), aluminum (Al), silver (Ag), Ag/ITO, Ag/IZO, aluminum-lithium(Al—Li), calcium (Ca), magnesium-indium (Mg—In), and magnesium-silver(Mg—Ag).

The metal paste may include any one of silver paste (Ag paste), aluminumpaste (Al paste), gold paste (Au paste), and copper paste (Cu paste), ormay be in the form of an alloy.

The metal ink material may include at least one of silver (Ag) ink,aluminum (Al) ink, gold (Au) ink, calcium (Ca) ink, magnesium (Mg) ink,lithium (Li) ink, and cesium (Cs) ink, and the metal material containedin the metal ink material may be ionized in the solution.

The positive electrode 110 may be formed on the substrate by aconventional vacuum deposition process (e.g., chemical vapor deposition,CVD) or an application method in which printing is performed using pastemetal ink prepared by mixing metal flakes or metal particles and abinder, and any method capable of forming an electrode may be usedwithout being limited to the above methods.

The hole injection layer 120 serves to transfer holes injected from thepositive electrode 110 to the hole transport layer 130, and is formedbetween the hole transport layer 130 and the positive electrode 110.

The hole injection layer 120 may be formed using a solution process.Specifically, the hole injection layer 120 may be formed using any onesolution process selected from spin coating, slit dye coating, ink-jetprinting, spray coating, and dip coating.

Preferably, the hole injection layer 120 may be formed using spincoating. In spin coating, a certain amount of a solution is dropped ontoa substrate while rotating the substrate at high speed. At this time,coating is performed by centrifugal force applied to the solution.

Since the hole injection layer 120 is formed using the solution process,a large area process may be performed, process time may be shortened,and limitations on the semiconductor characteristics of the upper andlower electrodes (positive and negative electrodes) may be reduced.

The hole injection layer 120 may be formed by forming a film using asolution prepared by mixing a p-type oxide semiconductor represented byFormula 1 below instead of commonly used PEDOT:PSS and a solvent.Cu₂Sn_(2-X)S₃—(Ga_(X))₂O₃,  [Formula 1]

wherein X is greater than 0.2 and less than 1.5 (0.2<x<1.5).

When the p-type oxide semiconductor is formed, a precursor solutioncontaining Cu, S, M, and Ga may be prepared (here, M includes one ormore compounds selected from SnO, ITO, IZTO, IGZO, and IZO), a coatinglayer may be formed by applying the precursor solution onto thesubstrate on which the positive electrode is formed, and then thecoating layer may be heat-treated.

The p-type oxide semiconductor may be heat-treated or treated withultraviolet light/ozone.

The heat treatment may be performed at 150 to 250° C. for 10 to 60minutes.

The ultraviolet light/ozone treatment may be performed for 0 to 5minutes.

The solvent may be prepared by mixing ethylene glycol and 5 to 50 volumepercent of at least one of 2-methoxyethanol, acetonitrile, DI water, analcohol, cyclohexane, toluene, and an organic solvent.

The hole transport layer 130 serves to transfer holes injected from thehole injection layer 120 to the light-emitting layer 140, and is formedbetween the hole injection layer 120 and the light-emitting layer 140.

The hole transport layer 130 may be formed by a vacuum depositionprocess using an organic material.

Specifically, the hole transport layer 130 may be formed by at least oneprocess of chemical vapor deposition, physical vapor deposition, atomiclayer deposition, metal organic chemical vapor deposition,plasma-enhanced chemical vapor deposition, molecular beam epitaxy,hydride vapor phase epitaxy, and sputtering. However, the presentdisclosure is not limited thereto, and other known methods may be used.

In the light-emitting layer 140, holes injected from the positiveelectrode 110 and passed through the hole transport layer and electronsinjected from the negative electrode 160 and passed through the electrontransport layer are recombined to generate excitons, and light isemitted when the generated excitons change from an excited state to aground state. In this case, the light-emitting layer 140 may be asingle-layer or multilayer form.

The light-emitting layer 140 may be formed by at least one process ofsputtering, spin coating, slit dye coating, ink-jet printing, spraycoating, dip coating, vacuum deposition, chemical vapor deposition,physical vapor deposition, atomic layer deposition, metal organicchemical vapor deposition, plasma-enhanced chemical vapor deposition,molecular beam epitaxy, and hydride vapor phase epitaxy. However, thepresent disclosure is not limited thereto, and other known methods maybe used.

The electron transport layer 150 serves to transfer electrons injectedfrom the negative electrode 160 to the light-emitting layer, and isformed between the light-emitting layer 140 and the negative electrode160.

The electron transport layer 150 may be formed by at least one processof sputtering, spin coating, slit dye coating, ink-jet printing, spraycoating, dip coating, vacuum deposition, chemical vapor deposition,physical vapor deposition, atomic layer deposition, metal organicchemical vapor deposition, plasma-enhanced chemical vapor deposition,molecular beam epitaxy, and hydride vapor phase epitaxy. However, thepresent disclosure is not limited thereto, and other known methods maybe used.

The negative electrode 160 is an electrode for providing electrons to adevice, and may be formed by performing a solution process such asscreen printing on a transmissive electrode, a reflective electrode, ametal paste, or a metal ink material in a colloid state in apredetermined liquid.

The transmissive electrode material may include at least one of indiumtin oxide (ITO), indium zinc oxide (IZO), tin oxide (SnO₂), zinc oxide(ZnO), multilayer metal oxide/metal/metal oxide, graphene, and carbonnanotube, which are transparent and have excellent conductivity.

The reflective electrode material may include at least one of magnesium(Mg), aluminum (Al), silver (Ag), Ag/ITO, Ag/IZO, aluminum-lithium(Al—Li), calcium (Ca), magnesium-indium (Mg—In), and magnesium-silver(Mg—Ag).

The metal paste may include any one of silver paste (Ag paste), aluminumpaste (Al paste), gold paste (Au paste), and copper paste (Cu paste), ormay be in the form of an alloy.

The metal ink material may include at least one of silver (Ag) ink,aluminum (Al) ink, gold (Au) ink, calcium (Ca) ink, magnesium (Mg) ink,lithium (Li) ink, and cesium (Cs) ink, and the metal material containedin the metal ink material may be ionized in the solution.

The negative electrode 160 may be formed on the substrate by aconventional vacuum deposition process (e.g., chemical vapor deposition,CVD) or an application method in which printing is performed using pastemetal ink prepared by mixing metal flakes or metal particles and abinder, and any method capable of forming an electrode may be usedwithout being limited to the above methods.

In the quantum dot light-emitting diode according to an embodiment ofthe present disclosure, a p-type oxide semiconductor containingCu₂SnS₃—Ga₂O₃ having high hole mobility and a high work function is usedas a hole injection layer. Thus, the electrical properties of thequantum dot light-emitting diode may be improved.

Hereinafter, the characteristics of quantum dot light-emitting diodesaccording to embodiments of the present disclosure will be describedwith reference to FIGS. 2A to 15B.

EXAMPLES

Preparation of Cu₂SnS₃—Ga₂O₃ Solutions

Copper (II) acetate monohydrate, tin (II) chloride, thiourea, andgallium (III) nitrate hydrate were added to 5 mL of a 2-methoxyethanolsolvent and stirred at 60° C. for 6 hours to prepare Cu₂SnS₃—Ga₂O₃solutions.

The Cu₂SnS₃—Ga₂O₃ solutions having various molar ratios of Sn to Ga wereprepared, and the composition ratios of Cu, Sn, S, and Ga precursorscontained in the Cu₂SnS₃—Ga₂O₃ solutions are shown in Table 1 below.

TABLE 1 0.1M Cu₂SnS₃-Ga₂O₃ in 2-methoxyethenol (5 ml) Cu:Sn:S:Ga1:1:8:1:0:2 1:1:5:1:0:5 1:1:1:1 1:0:5:1:1:5 Copper (II) 100 mg 100 mg100 mg 100 mg acetate (0.1M)  (0.1M)  (0.1M)  (0.1M)  monohydrate Tin(II) chloride 171 mg 143 mg  95 mg  48 mg (0.18M) (0.15M) (0.1M) (0.05M) Thiourea  38 mg  38 mg  38 mg  38 mg (0.1M)  (0.1M)  (0.1M) (0.1M)  Gallium (III)  26 mg  64 mg 128 mg 192 mg nitrate hydrate(0.02M) (0.05M) (0.1M)  (0.15M)

Fabrication of Quantum Dot Light-Emitting Diode

An ITO substrate having a sheet resistance of 9 Ωsq² was subjected toultrasonic treatment for 15 minutes in acetone, methanol, andisopropanol, respectively, followed by ultraviolet light/ozone treatmentfor 15 minutes.

Thereafter, spin coating was performed to coat the ITO substrate withCu₂SnS₃—Ga₂O₃, and the ITO substrate was treated with ultravioletlight/ozone (main wavelengths 185 and 254 nm) at 100° C. for 2 minutes.Then, the substrate was heat-treated under a nitrogen atmosphere to forma 30 nm hole injection layer.

A quantum dot light-emitting diode was fabricated by sequentiallydepositing PVK (15 nm) as a hole transport layer, green quantum dots asa light-emitting layer, LZO (70 nm) as an electron transport layer, andAl (100 nm) as a negative electrode on the hole injection layer.

In FIGS. 2A to 2K, the optical properties of a Cu₂SnS₃—Ga₂O₃ thin filmaccording to one embodiment of the present disclosure are shown.

FIG. 2A is a graph showing Raman spectra for an untreated Cu₂SnS₃—Ga₂O₃thin film (black line) and an ultraviolet light/ozone-treatedCu₂SnS₃—Ga₂O₃ thin film (gray line).

Referring to FIG. 2A, Raman peaks are observed at 297 cm⁻¹, 338 cm⁻¹,and 475 cm⁻¹, respectively.

Cu₂SnS₃ is known to have Raman peaks at 297 cm⁻¹ and 338 cm⁻¹. When aCuS phase is present in the Cu₂SnS₃—Ga₂O₃ thin film, Raman peaks arealso observed at 475 cm¹. Accordingly, based on Raman peak patterns, itcan be confirmed that Cu₂SnS₃ is contained in the thin film.

When a thin film is treated with ultraviolet light/ozone, the atomicstructure of a metal oxide and the composition of local electrons may bechanged.

Both untreated Cu₂SnS₃—Ga₂O₃ thin films and ultravioletlight/ozone-treated Cu₂SnS₃—Ga₂O₃ thin films have Raman peaks at 297cm⁻¹, 338 cm⁻¹, and 475 cm¹. These results indicate that there is nochange in the oxidation state of the Cu₂SnS₃—Ga₂O₃ thin film due toultraviolet light/ozone treatment.

FIG. 2B is a graph showing the X-ray diffraction (XRD) patterns ofCu₂SnS₃—Ga₂O₃ thin films heat-treated at 160° C. (black) and 200° C.(gray), respectively.

Referring to FIG. 2B, it can be seen that the Cu₂SnS₃—Ga₂O₃ thin filmheat-treated at 160° C. exhibits an amorphous phase, and theCu₂SnS₃—Ga₂O₃ thin film heat-treated at 200° C. exhibits peaks at 28.8°,47.6°, and 55.9° due to the (112), (204), and (312) crystal faces ofCu₂SnS₃.

From these results, it can be seen that the Cu₂SnS₃—Ga₂O₃ thin filmheat-treated at 200° C. is composed of two phases of crystalline Cu₂SnS₃and amorphous Ga₂O₃.

FIG. 2C is a graph showing the light transmittance of a Cu₂SnS₃—Ga₂O₃thin film depending on thickness, and FIG. 2D shows Tauc plots formeasuring band gaps.

Referring to FIG. 2C, it can be seen that, as the thickness of theCu₂SnS₃—Ga₂O₃ thin film is increased from 30 nm (black) to 90 nm (bottomdashed line), the transmittance at 550 nm is reduced from 96.1% to89.3%.

Transmittance may be reduced due to defects in band gaps or increase insurface roughness with increasing thickness, and the surface roughnessis related to transmittance due to diffused light.

Referring to FIG. 2D, it can be seen that the Tauc plots of(ahv)²-photon energy (hv) for Cu₂SnS₃—Ga₂O₃ thin films depending onthickness exhibit 3.84 eV to 3.86 eV.

FIG. 2E is a graph showing the light transmittance of a Cu₂SnS₃—Ga₂O₃thin film depending on the presence or absence of ultravioletlight/ozone treatment, and FIG. 2F shows Tauc plots for measuring bandgaps.

Referring to FIG. 2E, it can be seen that the light transmittances ofthe untreated Cu₂SnS₃—Ga₂O₃ thin film (black line) and the ultravioletlight/ozone-treated Cu₂SnS₃—Ga₂O₃ thin film (gray line) are more than90% in the visible light region.

Referring to FIG. 2F, the band gap of the ultravioletlight/ozone-treated Cu₂SnS₃—Ga₂O₃ thin film (gray line) is 3.70 eVdl,and the band gap of the untreated Cu₂SnS₃—Ga₂O₃ thin film (black line)is 3.86 eV. From these results, it can be confirmed that the band gap ofthe ultraviolet light/ozone-treated Cu₂SnS₃—Ga₂O₃ thin film is lowerthan that of the untreated Cu₂SnS₃—Ga₂O₃ thin film.

Reduction of a band gap by ultraviolet light/ozone treatment is relatedto reduction of transmittance in the UV region. When ultravioletlight/ozone treatment is performed, particle size may be increased,thereby reducing a band gap.

FIG. 2G is a graph showing the light transmittances of Cu₂SnS₃—Ga₂O₃thin films having molar ratios of Sn to Ga (Sn:Ga) of 1.8:0.2, 1.5:0.5,1.0:1.0, and 0.5:1.5, respectively, and FIG. 2H shows Tauc plots formeasuring band gaps.

Referring to FIG. 2G, it can be seen that the Cu₂SnS₃—Ga₂O₃ thin filmshaving molar ratios of Sn to Ga (Sn:Ga) of 1.5:0.5, 1.0:1.0, and0.5:1.5, respectively, exhibit a light transmittance of 85% or more inthe visible light region, and the Cu₂SnS₃—Ga₂O₃ thin film having a molarratio of Sn to Ga (Sn:Ga) of 1.8:0.2 exhibits a relatively low lighttransmittance.

Referring to FIG. 2H, it can be seen that the Cu₂SnS₃—Ga₂O₃ thin filmshaving molar ratios of Sn to Ga (Sn:Ga) of 1.8:0.2, 1.5:0.5, 1.0:1.0,and 0.5:1.5, respectively, have band gaps of 3.23 eV, 3.53 eV, 3.70 eV,and 3.86 eV, respectively.

FIG. 2I is an atomic force microscope (AFM) image of a Cu₂SnS₃—Ga₂O₃thin film, FIG. 2J is a scanning electron microscope (SEM) image of aCu₂SnS₃—Ga₂O₃ thin film, FIG. 2K is a high-resolution transmissionelectron microscope (HRTEM) image of a Cu₂SnS₃—Ga₂O₃ thin film, and FIG.2L shows a crystal structure of a selected region obtained by electrondiffraction.

Referring to FIGS. 2I to 2L, it can be seen that the Cu₂SnS₃—Ga₂O₃ thinfilm is composed of two phases of crystalline Cu₂SnS₃ and amorphousGa₂O₃.

FIG. 3A shows energy-dispersive spectrometer (EDX) spectra for analyzingthe chemical composition of a Cu₂SnS₃—Ga₂O₃ thin film according to anembodiment of the present disclosure.

Referring to FIG. 3A, it can be seen that Cu, Sn, S, Ga, and O arecontained in the Cu₂SnS₃—Ga₂O₃ thin film.

FIGS. 3B to 3F are graphs showing the results of X-ray photoelectronspectroscopy (XPS) analysis for analyzing the chemical composition ofthe surface of a Cu₂SnS₃—Ga₂O₃ thin film treated with ultravioletlight/ozone for 2 minutes and heat-treated at 200° C.

Referring to FIG. 3B, the Cu 2p spectrum exhibits a strong peak of Cu2_(3/2) at 932.7 eV and a strong peak of Cu 2p_(1/2) at 952.5 eV. Fromthese results, it can be confirmed that Cu₂SnS₃ is present.

Referring to FIG. 3C, in the Sn 3d spectrum, four different peaks, i.e.,two peaks of Sn²⁺ (495.2 eV) and Sn⁴⁺ (495.6 eV) for Sn 3d_(3/2) and twopeaks of Sn²⁺ (486.8 eV) and Sn⁴⁺ (487.3 eV) for Sn 3d_(5/2), arefitted.

At about 485.2 eV, Sn metal is not detected, indicating that Sn ispresent in the form of Cu₂SnS₃.

Referring to FIG. 3D, it can be seen that, in the S 2p spectrum, threedistinct peaks are observed at 161.0 eV, 162.6 eV, and 164.0 eV,respectively, by a metal sulfide, CuS, and CuS₂. Based on these results,the chemical composition of Cu₂SnS₃ can be determined.

Referring to FIG. 3E, in the Ga 2p spectrum, peaks of 1118.4 eV and1145.3 eV corresponding to Ga2p_(3/2) and Ga2p_(1/2), respectively, areobserved. From the binding energy, it can be seen that Ga is in atrivalent oxidation state (Ga³⁺).

From these results, it can be seen that Ga₂O₃ is contained in theCu₂SnS₃—Ga₂O₃ thin film.

Referring to FIG. 3F, in the O 1s spectrum, two compositionscorresponding to Ga—O (531.0 eV) and Ga—OH (532.4 eV), respectively, areobserved, indicating that Ga₂O₃ is present.

FIGS. 4A to 4C are contour plots for carrier concentration, Hallmobility, and resistivity depending on heat treatment temperature andheat treatment time in a Cu₂SnS₃—Ga₂O₃ thin film treated withultraviolet light/ozone for 2 minutes according to an embodiment of thepresent disclosure, and FIGS. 4D to 4F are error bar plots for carrierconcentration, Hall mobility, and resistivity.

When heat treatment is performed at a temperature of less than 160° C.and greater than 220° C., measurement is impossible due to the highresistance of the Cu₂SnS₃—Ga₂O₃ thin film.

The carrier concentration, Hall mobility, and resistivity of theCu₂SnS₃—Ga₂O₃ thin film depending on heat treatment temperature and heattreatment time are shown in Table 2 below.

TABLE 2 Heat treatment temperature 160° C. 180° C. 200° C. 220° C.Carries concentration (×10¹⁷ cm⁻³) Heat 10 min 4.42 ± 1.02 0.99 ± 0.030.87 ± 0.03 0.16 ± 0.06 treatment 30 min 1.92 ± 0.90 0.75 ± 0.04 0.66 ±0.04 0.12 ± 0.07 time 60 min 0.86 ± 0.08 0.66 ± 0.03 0.58 ± 0.03 0.08 ±0.06 90 min 0.30 ± 0.01 0.10 ± 0.01 0.09 ± 0.01 0.02 ± 0.01 Hallmobility (cm²/Vs) Heat 10 min 1.34 ± 0.96 1.59 ± 1.14 3.87 ± 1.31 0.78 ±0.52 treatment 30 min 5.24 ± 2.52 6.21 ± 2.99 9.78 ± 2.68 4.05 ± 1.68time 60 min 11.42 ± 2.27  22.55 ± 1.70  25.77 ± 2.21  15.33 ± 0.78  90min 6.64 ± 1.56 7.87 ± 1.60 20.87 ± 2.56  10.20 ± 2.86  Resistivity (Ωcm) Heat 10 min 422.22 ± 32.21  415.10 ± 24.45  310.75 ± 25.24  381.44 ±33.45  treatment 30 min 315.16 ± 23.11  215.65 ± 23.54  67.24 ± 72.25197.72 ± 125.62 time 60 min 98.60 ± 19.15 83.60 ± 70.75 4.02 ± 0.3724.74 ± 1.78  90 min 121.21 ± 17.35  109.04 ± 28.12  5.07 ± 1.14 30.77 ±6.84 

Referring to FIGS. 4A to 4F and Table 2, although the band gap is about3.7 eV, the carrier concentration is larger than 10¹⁶ cm⁻³, and the Hallmobility is increased as heat treatment temperature and heat treatmenttime are increased.

In addition, it can be confirmed that the Cu₂SnS₃—Ga₂O₃ thin film is ap-type oxide semiconductor based on the result that the Cu₂SnS₃—Ga₂O₃thin film has a positive carrier concentration value.

In addition, it can be seen that, when heat treatment temperature is200° C. and heat treatment time is 60 minutes, the highest hole mobilityand the lowest resistivity are observed.

FIGS. 5A to 5C are contour plots for carrier concentration, Hallmobility, and resistivity depending on ultraviolet light/ozone treatmenttime and molar ratios of Sn to Ga (Sn:Ga) in a Cu₂SnS₃—Ga₂O₃ thin filmheat-treated at 200° C. for 60 minutes according to an embodiment of thepresent disclosure, and FIGS. 5D to 5F are error bar plots for carrierconcentration, Hall mobility, and resistivity.

The carrier concentration, Hall mobility, and resistivity of theCu₂SnS₃—Ga₂O₃ thin film depending on ultraviolet light/ozone treatmenttime and molar ratios of Sn to Ga (Sn:Ga) are shown in Table 3 below.

TABLE 3 Ultraviolet light/ozone treatment time 0 min 2 min 3 min 5 minCarries concentration (×10¹⁷ cm⁻³) Molar ratio 1.8:0.2 0.88 ± 0.07 3.86± 0.31 4.54 ± 0.37 5.09 ± 0.30 of Sn:Ga 1.5:0.5 0.16 ± 0.01 0.70 ± 0.020.94 ± 0.07 3.01 ± 0.32 1.0:1.0 0.15 ± 0.01 0.68 ± 0.03 0.80 ± 0.04 1.08± 0.30 0.5:1.5 0.13 ± 0.01 0.58 ± 0.04 0.69 ± 0.04 0.82 ± 0.04 Hallmobility (cm²/Vs) Molar ratio 1.8:0.2 0.56 ± 0.16 14.00 ± 3.16  12.26 ±3.54  9.90 ± 2.86 of Sn:Ga 1.5:0.5 0.83 ± 0.16 20.10 ± 2.86  18.28 ±3.60  14.76 ± 2.90  1.0:1.0 1.03 ± 0.13 25.63 ± 3.11  22.65 ± 2.94 18.28 ± 2.37  0.5:1.5 1.43 ± 0.10 36.34 ± 1.92  31.37 ± 2.18  25.32 ±1.75  Resistivity (Ω cm) Molar ratio 1.8:0.2 183.80 ± 71.5  32.21 ±6.78  200.80 ± 27.96  553.06 ± 95.8  of Sn:Ga 1.5:0.5 488.15 ± 93.41 12.32 ± 4.15  99.80 ± 23.68 210.43 ± 70.83  1.0:1.0 1298 ± 437  4.63 ±1.95 51.20 ± 16.3  79.07 ± 33.40 0.5:1.5  1670 ± 48.44 1.88 ± 1.32 20.60± 11.1  32.07 ± 22.47

Referring to FIGS. 5A to 5F and Table 3, when ultraviolet light/ozonetreatment is not performed, the resistivity of the Cu₂SnS₃—Ga₂O₃ thinfilm is gradually increased as the molar ratio of Ga is increased. Onthe other hand, when ultraviolet light/ozone treatment is performed, theresistivity of the Cu₂SnS₃—Ga₂O₃ thin film is gradually decreased as themolar ratio of Ga is increased.

These results are obtained due to the high dielectric properties ofGa₂O₃ heat-treated under a nitrogen atmosphere.

When ultraviolet light/ozone treatment time is increased from 0 minutesto 2 minutes, the Hall mobility is increased and the resistivity isdecreased. When ultraviolet light/ozone treatment time is furtherincreased, the Hall mobility is decreased and the resistivity isincreased.

Based on these results, it can be seen that, when the molar ratio of Snto Ga (Sn:Ga) is 0.5:1.5 and ultraviolet light/ozone treatment isperformed for 2 minutes, the Cu₂SnS₃—Ga₂O₃ thin film has excellentelectrical properties.

FIGS. 6A and 6B show a schematic configuration of a Cu₂SnS₃—Ga₂O₃ thinfilm according to an embodiment of the present disclosure consisting ofcrystalline Cu₂SnS₃ and amorphous Ga₂O₃, and show current flow throughthe Cu₂SnS₃—Ga₂O₃ thin film.

Referring to FIGS. 6A and 6B, crystalline Ga₂O₃ is known as a conductiveoxide having a wide band gap, and amorphous Ga₂O₃ is an insulatingmaterial. Accordingly, current present in the Cu₂SnS₃—Ga₂O₃ thin film isgenerated through crystalline Ga₂O₃.

FIGS. 7A to 7D show the He (I) ultraviolet photoelectron spectroscopy(UPS) spectra of PEDOT:PSS or a Cu₂SnS₃—Ga₂O₃ thin film deposited on anITO substrate according to an embodiment of the present disclosure.

Referring to FIG. 7A, it can be seen that the work functions of theuntreated Cu₂SnS₃—Ga₂O₃ thin film, the ultraviolet light/ozone-treatedCu₂SnS₃—Ga₂O₃ thin film, and PEDOT:PSS can be obtained by the interceptsbetween the extrapolation of the leading edge and the extrapolationlevel in a high binding energy region.

According to the secondary electron cut-offs of the untreatedCu₂SnS₃—Ga₂O₃ thin film, the ultraviolet light/ozone-treatedCu₂SnS₃—Ga₂O₃ thin film, and PEDOT:PSS, work functions thereof are 4.51eV, 4.92 eV, and 5.17 eV, respectively, and the work function of ITO isabout 4.2 eV.

Referring to FIGS. 7B to 7D, it can be seen that the VB edges of theuntreated Cu₂SnS₃—Ga₂O₃ thin film, the ultraviolet light/ozone-treatedCu₂SnS₃—Ga₂O₃ thin film, and PEDOT:PSS are 0.59 eV, 0.38 eV, and 0.24eV, respectively, under the Fermi level of ITO.

The ionization potential (E_(ion)) can be obtained by summing the workfunction and the VB edge energy. The ionization potentials of theuntreated Cu₂SnS₃—Ga₂O₃ thin film, the ultraviolet light/ozone-treatedCu₂SnS₃—Ga₂O₃ thin film, and PEDOT:PSS are as follows.E_(ion) (Untreated Cu₂SnS₃—Ga₂O₃ thin film)=4.92+0.38=5.30 eVE_(ion) (Ultraviolet light/ozone-treated Cu₂SnS₃—Ga₂O₃ thinfilm)=5.17+0.24=5.41 eVE_(ion) (PEDOT:PSS)=4.51+0.59=5.10 eV

From the above calculations, it can be seen that the ionizationpotential (E_(ion)) of the Cu₂SnS₃—Ga₂O₃ thin film is larger than thatof PEDOT:PSS. These results indicate that the Cu₂SnS₃—Ga₂O₃ thin film issuitable as the hole injection layer of a quantum dot diode.

FIG. 8A shows UPS spectra in the secondary electron cut-off regions of aCu₂SnS₃—Ga₂O₃ thin film measured at various negative biases, and FIG. 8Bshows UPS spectra in the VB edges of the Cu₂SnS₃—Ga₂O₃ thin film.

Referring to FIG. 8A, it can be seen that the secondary electron cut-offregion shifts to higher binding energy as the negative bias isincreased.

Referring to FIG. 8B, it can be seen that, at various negative biases,the VB edge from the work function of the Cu₂SnS₃—Ga₂O₃ thin film isindependent of the negative biases.

FIG. 9A is a cross-sectional view of a quantum dot light-emitting diodeaccording to an embodiment of the present disclosure, and FIG. 9B is across-sectional view of a TEM image of the quantum dot light-emittingdiode.

FIG. 9C is an HRTEM image of green quantum dots according to anembodiment of the present disclosure, and FIG. 9D illustrates energyband diagrams of quantum dot light-emitting diodes including varioushole injection layers.

Referring to FIGS. 9A and 9B, it can be seen that the green quantum dotlight-emitting diode has a structure of ITO/PEDOT:PSS (30 nm)/PVK (15nm)/green quantum dots/Li doping ZnO (LZO) (70 nm)/Al (100 nm).

CdSe/CdS/ZnS green quantum dots are used as a light-emitting layer, andPVK and LZO are used as a hole transport layer and an electron transportlayer, respectively.

Referring to FIGS. 9A to 9D, it can be seen that the Cu₂SnS₃—Ga₂O₃ thinfilm is uniformly deposited on an ITO substrate, and the surface of theCu₂SnS₃—Ga₂O₃ thin film is smoothly formed.

FIG. 10A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode including aCu₂SnS₃—Ga₂O₃ thin film according to an embodiment of the presentdisclosure depending on ultraviolet light/ozone treatment time, FIG. 10Bis a graph showing the luminance-voltage (L-V) characteristics of thequantum dot light-emitting diode, and FIG. 10C is a graph showing thecurrent efficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode.

Referring to FIGS. 10A to 10C, it can be seen that current density isdecreased with increasing ultraviolet light/ozone treatment time from 2minutes to 3 minutes. Decrease in current density is caused by increasein the resistivity of the Cu₂SnS₃—Ga₂O₃ thin film, and the quantum dotlight-emitting diode exhibits optimal performance when ultravioletlight/ozone treatment time is 2 minutes.

FIG. 11A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode including aCu₂SnS₃—Ga₂O₃ thin film according to an embodiment of the presentdisclosure depending on heat treatment temperatures, FIG. 11B is a graphshowing the luminance-voltage (L-V) characteristics of the quantum dotlight-emitting diode, and FIG. 11C is a graph showing the currentefficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode.

Referring to FIGS. 11A to 11C, it can be seen that, as heat treatmenttemperature is increased to 200° C., current density is also increased,and maximum current density is exhibited when heat treatment temperatureis 200° C.

Accordingly, it can be seen that, in the case of a quantum dotlight-emitting diode including a Cu₂SnS₃—Ga₂O₃ thin film heat-treated at200° C., the current density-voltage (J-V) characteristics,luminance-voltage (L-V) characteristics, current efficiency-luminance(C/E-L) characteristics, and power efficiency-luminance characteristicsthereof are improved.

FIG. 12A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode including aCu₂SnS₃—Ga₂O₃ thin film according to an embodiment of the presentdisclosure depending on molar ratios of Sn:Ga, FIG. 12B is a graphshowing the luminance-voltage (L-V) characteristics of the quantum dotlight-emitting diode, and FIG. 12C is a graph showing the currentefficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode.

Referring to FIGS. 12A to 12C, as the molar ratio of Ga is increased to1.5, current density and luminance are increased. This is due todecrease in the resistivity of the Cu₂SnS₃—Ga₂O₃ thin film.

However, when the molar ratio of Sn to Ga (Sn:Ga) is 0.5:1.5, chargecarriers in the light-emitting layer are unbalanced due to therelatively low resistivity of the Cu₂SnS₃—Ga₂O₃ thin film. As a result,current efficiency is reduced.

FIG. 13A is a graph showing the current density-voltage (J-V)characteristics of a quantum dot light-emitting diode according to anembodiment of the present disclosure depending on the thicknesses of aCu₂SnS₃—Ga₂O₃ thin film, FIG. 13B is a graph showing theluminance-voltage (L-V) characteristics of the quantum dotlight-emitting diode, and FIG. 13C is a graph showing the currentefficiency-luminance (C/E-L) characteristics of the quantum dotlight-emitting diode.

Referring to FIGS. 13A to 13C, when the thickness of the Cu₂SnS₃—Ga₂O₃thin film is 15 nm or 30 nm, similar current efficiency is observed.When the thickness is 45 nm, current efficiency is dramatically reduced.

FIGS. 14A to 14E are graphs showing the characteristics of a quantum dotlight-emitting diode including a PEDOT:PSS-based hole injection layerand a quantum dot light-emitting diode including a Cu₂SnS₃—Ga₂O₃ thinfilm-based hole injection layer having a molar ratio of Sn to Ga (Sn:Ga)of 1:1 according to an embodiment of the present disclosure.

FIG. 14A shows current density-voltage (J-V) characteristics, FIG. 14Bshows luminance-voltage (L-V) characteristics, and FIG. 14C showscurrent efficiency-luminance (C/E-L) characteristics.

FIG. 14D shows power efficiency-luminance characteristics, and FIG. 14Eshows external quantum efficiency-luminance characteristics.

In Table 4, details of the results of FIGS. 14A to 14E are shown.

TABLE 4 External Hole Driving quantum Current Power injection voltageLuminance efficiency efficiency efficiency layer (V) (cd m⁻²) (%) (cdA⁻¹) (lm W⁻¹) PEDOT:PSS 6.05 39110 12.36 42.66 20.33 Cu₂SnS₃- 4.64 7382014.93 51.72 31.97 Ga₂O₃ thin film

Referring to FIGS. 14A to 14E and Table 4, the quantum dotlight-emitting diode including a Cu₂SnS₃—Ga₂O₃ thin film-based holeinjection layer having a molar ratio of Sn to Ga (Sn:Ga) of 1:1 exhibitshigh current density. This data indicates that the hole injectioncapability of the Cu₂SnS₃—Ga₂O₃ thin film is superior to that ofPEDOT:PSS.

Compared to the quantum dot light-emitting diode including aPEDOT:PSS-based hole injection layer, the quantum dot light-emittingdiode including a Cu₂SnS₃—Ga₂O₃ thin film-based hole injection layerexhibits higher external quantum efficiency, current efficiency, andpower efficiency, indicating that hole injection through theCu₂SnS₃—Ga₂O₃ thin film is more efficient than PEDOT:PSS.

FIG. 15A is a graph showing EL intensity-current density depending onwavelength, and FIG. 15B shows a CIE color specification system.

Referring to FIG. 15A, the emission peak of the quantum dotlight-emitting diode including a PEDOT:PSS-based hole injection layer is527 nm, and the emission peak of the quantum dot light-emitting diodeincluding a Cu₂SnS₃—Ga₂O₃ thin film-based hole injection layer is 522nm. From these results, it can be seen that pure green light having aband gap of 22 nm is emitted.

Referring to FIG. 15B, based on the color coordinates (0.168, 0.788) ofthe quantum dot light-emitting diode including a Cu₂SnS₃—Ga₂O₃ thinfilm-based hole injection layer and the color coordinates (0.176, 0.785)of the quantum dot light-emitting diode including a PEDOT:PSS-based holeinjection layer, it can be seen that the quantum dot light-emittingdiode including a Cu₂SnS₃—Ga₂O₃ thin film-based hole injection layeremits light with a higher color saturation than that of the quantum dotlight-emitting diode including a PEDOT:PSS-based hole injection layer.

As described above, the quantum dot light-emitting diode of the presentdisclosure includes a Cu₂SnS₃—Ga₂O₃ thin film-based hole injection layerhaving high hole mobility, a high work function, and high transparency.Accordingly, the current density, luminance, external quantumefficiency, current efficiency, and power efficiency of the quantum dotlight-emitting diode of the present disclosure may be improved.

In addition to the hole injection layer of a quantum dot light-emittingdiode, the Cu₂SnS₃—Ga₂O₃ thin film may be used as the active layer of atransistor.

According to the embodiments of the present disclosure, by using ap-type oxide semiconductor including Cu₂SnS₃—Ga₂O₃ as a hole injectionlayer, a high-efficiency quantum dot light-emitting diode can befabricated.

According to the embodiments of the present disclosure, by using ap-type oxide semiconductor fabricated using a solution process, aquantum dot light-emitting diode applicable to a low-temperature processcan be fabricated, thereby reducing preparation costs.

Although the present disclosure has been described through limitedexamples and figures, the present disclosure is not intended to belimited to the examples. Those skilled in the art will appreciate thatvarious modifications, additions, and substitutions are possible,without departing from the scope and spirit of the invention.

Therefore, the scope of the present disclosure should not be limited bythe embodiments, but should be determined by the following claims andequivalents to the following claims.

DESCRIPTION OF SYMBOLS

-   100: QUANTUM DOT LIGHT-EMITTING DIODE-   110: POSITIVE ELECTRODE-   120: HOLE INJECTION LAYER-   130: HOLE TRANSPORT LAYER-   140: LIGHT-EMITTING LAYER-   150: ELECTRON TRANSPORT LAYER-   160: NEGATIVE ELECTRODE

What is claimed is:
 1. A method of fabricating a quantum dotlight-emitting diode, comprising: forming a positive electrode on asubstrate; forming a hole injection layer on the positive electrode;forming a hole transport layer on the hole injection layer; forming alight-emitting layer on the hole transport layer; forming an electrontransport layer on the light-emitting layer; and forming a negativeelectrode on the electron transport layer, wherein the hole injectionlayer is formed by forming a film using a solution prepared by mixing ap-type oxide semiconductor represented by Formula 1 below and a solvent:Cu₂Sn_(2-X)S₃—(Ga_(X))₂O₃,  [Formula 1] wherein X is greater than 0.2and less than 1.5 (0.2<x<1.5).
 2. The method according to claim 1,wherein forming the p-type oxide semiconductor comprises preparing aprecursor solution containing Cu, S, M, and Ga, wherein M comprises oneor more compounds selected from SnO, ITO, IZTO, IGZO, and IZO; forming acoating layer by applying the precursor solution onto the substrate onwhich the positive electrode is formed; and heat-treating the coatinglayer.
 3. The method according to claim 1, wherein the solvent isprepared by mixing 2-methoxyethanol, ethylene glycol, and 5 to 50 volumepercent of at least one of acetonitrile, DI water, an alcohol,cyclohexane, toluene, and an organic solvent.
 4. The method according toclaim 1, wherein the p-type oxide semiconductor is heat-treated ortreated with ultraviolet light/ozone.
 5. The method according to claim4, wherein the heat treatment is performed at 150 to 250° C.
 6. Themethod according to claim 4, wherein the heat treatment is performed for10 to 90 minutes.
 7. The method according to claim 4, wherein theultraviolet light/ozone treatment is performed for 0 to 5 minutes.